Do read this as I think it correctly sums up the legalities
FHBVC newsletter In the real world, I have presented a car with LEDs for a MOT, but on mine, they are legal, yours isn't. But, the MOT tester does not take things to bits, so if he's not told - he won't know!
As a result, the sensible advice is fit ones you think are ok, to one side and compare with the other after dark. Don't just look directly behind, but also look at an angle. It is debatable whether LED brake lights are ever legal since some sources say the bulb wattage is specified and obviously, LEDs consume way less power. (But I have a defence as the car is pre 34...).